1. Field of the Invention
This invention relates to an apparatus for unwinding a wire material from a reel, spool or the like under constant tension.
2. Description of Related Art
In FIG. 1 is shown an embodiment of the conventional apparatus for unwinding a wire material under constant tension, in which a reel 20 winding a wire material 22 is placed on a spindle shaft 1 by inserting a support portion 2 of the shaft 1 into a hole 21 of the reel 20. The wire material 22 is unwound from the reel 20 through fixed rolls 10, 11 and a dancer roll 6 as shown by a phantom line. The dancer roll 6 is placed on an end of a rotatably arranged dancer arm 7. A cylindrical portion 13 is coaxially arranged on the dancer arm 7 with respect to the rotating axis thereof. A brake band 4 is connected at its one end to the dancer arm 7 and fixed at the other end to a fixed portion 12 through contacting with the cylindrical portion 13 and a braking face 3.
The dancer arm 7 is energized by a spring 8 fixed at its one end to a spring fixed portion 14 in a direction shown by an arrow X or in a direction of increasing a tension of the brake band 4 to increase a contact pressure to the braking face 3. On the other hand, the dancer arm 7 is subjected to a rotating force by the tension of the wire material 22 through the dancer roll 6 in a direction shown by an arrow Y or in a direction of decreasing the tension of the brake band 4 to decrease the contact pressure to the braking face 3. Thus, a braking force to the rotation of the spindle shaft 1 is controlled by a balance between the rotating force and the energizing force through the spring 8.
That is, when the tension of the wire material 22 decreases during the unwinding of the wire material 22, the dancer arm 7 rotates in the direction of arrow X and hence the tension of the bake band 4 and the contact pressure to the braking face 3 are increased to increase the tension of the wire material 22. On the other hand, when the tension of the wire material 22 increases, the dancer arm 7 rotates in the direction of arrow Y and hence the tension of the bake band 4 and the contact pressure to the braking face 3 are decreased to decrease the tension of the wire material 22.
In the conventional apparatus, therefore, it is desired to maintain the unwinding tension of the wire material at an approximately constant value by controlling the change of tension in the wire material to be unwound through the above construction and function.
However, when the dancer arm 7 rotates in the direction of arrow Y to decrease the tension of the brake band 4, there is caused a problem that the looseness of the brake band 4 is caused and the control of the unwinding tension is not conducted smoothly. Particularly, this problem is apt to be caused when the tension of the brake band 4 is set to a relatively low value for unwinding the wire material under a low tension.
In FIG. 1, a phantom line W1 shows a state of loosing the brake band 4 to form a curved deformation in the vicinity of the contact portion with the braking face 3. As such a curved deformation is created, the contact area to the braking face 3 decreases to excessively decrease the braking force and hence there is caused a problem that the tension change of the wire material 22 becomes large. Also as the braking force excessively decreases, the tension of the brake band 4 rapidly increases to increase the tension of the wire material 22 and hence there is caused a problem that breakage or hunting phenomenon of the wire material occurs.
On the other hand, when the brake band 4 is curvedly deformed in the vicinity of a connected portion to the dancer arm 7 as shown by a phantom line W2, the brake band 4 rises to the cylindrical portion 13 and it is difficult to reflect the movement of the dancer arm 7 to the change of the braking force and hence the tension of the wire material 22 becomes not smoothly controlled.